It has long been recognized that the angular pitch of a propeller's blades is significant in determining the efficient operation of the propeller propulsion system, whether for a boat or an aircraft. For marine propellers, propeller blade pitch is often defined in terms of "inches", i.e., defining the distance that a boat would be propelled through the water by a single revolution of the propeller, assuming no slippage, e.g. a propeller having a pitch of "13 inches", is one having the blade angle necessary to linearly advance the boat 13 inches upon one complete revolution of the propeller.
It has similarly been well understood that the conditions under which the boat will operate are important in determining the optimum pitch for the propeller, for an engine producing a certain maximum power output Such operating conditions include the load, intended speed of the boat, and the type of hull, of the boat being propelled. In the past, the majority of boat propulsion systems whether inboard, outboard or stern drive, have been designed to use a single pitch propeller, wherein the propeller was changed depending upon the boat to which the engine would be attached, and its intended use. For example, when a boat was to be used for towing a water-skier, i.e where the boat is subjected to a relatively heavy load, a propeller having a lower pitch would be selected, e.g. approximately a 15" pitch for a relatively small, 16 feet long outdoor pleasure boat with a 100 h.p. engine. Similarly, a higher speed boat with, e.g. a 300 h.p. engine would use a relatively high pitch blade, e.g. a 21"-pitch propeller
Designers have also long recognized that such fixed pitch propellers are at best compromises which produce fully acceptable performance only over a relatively narrow range of operating conditions, e.g. either slow speed or fast speed operation, but not both.
Past workers have designed propellers which have manually resettable blade pitch positions. Thus, the pitch of the propeller can be modified based upon the anticipated overall use to which a boat will next be put; however, such pitch was set before starting the engine, and the pitch remained constant during continued engine operation. Thus, if a boat was to be used for water-skiing, a relatively lower pitch would be selected and the boat operated at that constant pitch during the entire operating session. A lower pitch propeller permits the engine to operate at a high rotary speed, and thus develop large amounts of power at a relatively low boat speed. However, the constant lower pitch reduces the effective top cruising speed. Such a device is shown for example in U.S. Pat. No. 3,790,304. Other past designs have manually resettable blade positions that allow changes in the blade pitch position during operation. These have provided for manual adjustments made via mechanical, hydraulic or electric means. Such devices are shown for example in U.S. Pats. No. 2,554,716, 3,216,507, and 4,599,043.
The prior art, recognizing the utility of propellers which vary blade pitch during operation of the engine, have devised various means of changing the pitch either in accordance with a self-actuating design, i.e. the pitch automatically changes based upon changes in operating conditions, e.g., engine RPM, or by operator-controlled means, such as pneumatic or hydraulic controllers. Self-actuating propellers, which are apparently continuously variable over a range of pitch positions, are suggested for marine propellers by Reid in U.S. Pat. No. 3,177,948, and for aircraft propellers by Lagrevol and Biermann, in U.S. Pat. Nos. 2,669,311 and 2,694,459. A propeller, especially adapted for an outboard engine for marine use, having both manual and automatic self-actuating variable pitch means, is shown in U.S. Pat. No. 2,682,926, to Evans.
Other devices which provide for automatic, self-actuated changes in blade pitch positions, wherein the blades are spring biased against change, is shown for example in U.S. Pat. Nos. 2,290,666, 2,988,156; 3,145,780; 3,204,702; 3,229,772; 3,231,023; 3,295,610; and 3,567,336. In addition, there have been variable pitch marine propeller designs which are actuated by a sudden, or sharp, change in engine RPM to provide the necessary impetus to shift the blade pitch. Examples of such devices are shown in U.S. Pat. Nos. 3,275,083 and 3,302,725.
Prior self-actuating propellers intended primarily for uses on aircraft have incorporated means to lock the blades in one or more blade positions. Such devices are shown for example in U.S. Pat. Nos. 2,669,311 and 2,694,459, and German Patent publication No. DE 3,429,297.